fix: add permanent session deletion

[SuperstructureJH]

Summary

• add a destructive Delete session action with a confirmation dialog that clearly preserves workspace files
• add the session.delete RPC across the desktop client, web connection, host proxy, remote-agent ownership, workspace registry, and JSONL persistence layers
• stop and drain agents owned by the deleting host, reject subagent-owned or externally busy sessions, and synchronize removal to peer clients
• ship the changes as reproducible patch-package overlays for the pinned Harness 0.1.0-rc.6 packages
• add regression coverage for patch application and durable JSONL deletion

Validation

• built and signed a macOS arm64 DMG
• manually verified permanent session deletion in the packaged desktop app
• confirmed workspace files are retained after deleting the session